1. Field of the Invention
The present invention relates to a nitride-based semiconductor light emitting diode (LED). In the nitride-based semiconductor LED, an area around a p-electrode pad, in which light is preferentially emitted, is expanded so as to enhance light extraction efficiency, and local current crowding is prevented so as to reduce a driving voltage.
2. Description of the Related Art
Because group III-V nitride semiconductors such as GaN have excellent physical and chemical properties, they are considered as essential materials of light emitting devices, for example, light emitting diodes (LEDs) or laser diode (LDs). The LEDs or LDs formed of the group III-V nitride semiconductors are widely used in the light emitting devices for obtaining blue or green light. The light emitting devices are applied to light sources of various products, such as household appliances, electronic display boards, and lighting devices. Generally, the group III-V nitride semiconductors are comprised of gallium nitride (GaN) based materials having an compositional formula of InXAlYGa1-X-YN (0≦X, 0≦Y, X+Y≦1).
Hereinafter, a conventional nitride-based semiconductor LED will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is a sectional view illustrating the conventional nitride-based semiconductor LED, and FIG. 2 is a plan view illustrating the conventional nitride-based semiconductor LED.
As shown in FIG. 1, the nitride-based semiconductor LED 100 includes a sapphire substrate 101 for growing nitride-based semiconductor materials, an n-type nitride semiconductor layer 102, an active layer 103, and a p-type nitride semiconductor layer 104, which are sequentially formed on the sapphire substrate 101. Portions of the p-type nitride semiconductor layer 104 and the active layer 103 are removed by a mesa etching process, so that the n-type nitride semiconductor layer 102 is partially exposed.
On the p-type nitride semiconductor layer 104 which has not been etched by the mesa etching process, a p-electrode pad 106 is formed. On the n-type nitride semiconductor layer 102, an n-electrode pad 107 is formed.
Since the p-type nitride semiconductor layer 104 has larger specific resistance than the n-type nitride semiconductor layer 102, a difference in resistance between the p-type nitride semiconductor layer 104 and the n-type nitride semiconductor layer 102 causes a current spreading effect to be reduced. As such, when a current spreading effect decreases, light extraction efficiency also decreases so that the brightness of the nitride semiconductor LED 100 is reduced. Accordingly, in order to enhance a current spreading effect in the related art, a transparent electrode 105 is formed on the p-type nitride semiconductor layer 104 so as to increase an injection area of current which is injected through the p-electrode pad 106.
In the above-described nitride-based semiconductor LED 100, the transparent electrode 105 is further provided on the p-type nitride semiconductor 104 so as to obtain an enhanced current spreading effect. However, when a difference in surface resistance between the transparent electrode 105 and the n-type nitride semiconductor layer 102 is large, a current spreading effect is still small. For example, when a commonly-used ITO (indium tin oxide) is used as the transparent electrode 105, local current crowding occurs in the vicinity (refer to reference numeral ‘A1’) of the p-electrode pad because of high surface resistance of the ITO.
In the nitride-based semiconductor LED 100, the p-electrode pad 106 is formed as close to the outer edge line of the p-type nitride semiconductor layer 104 as possible, the outer edge line being a mesa line. Further, the p-electrode pad 106 and the n-electrode 107 is spaced at the maximum distance from each other so as to secure the maximum light emitting area therebetween. Then, an optical output is expected to be enhanced. In this case, however, local current crowding increases in the vicinity (A1) of the p-electrode pad 106, thereby degrading the reliability of the diode.
The vicinity (A1) of the p-electrode pad 106 is a region (hereinafter, referred to as ‘preferential light emission region’) in which light is preferentially emitted. When the p-electrode pad 106 is formed close to the mesa line, there is a limit in securing an area in the vicinity (A1) of the p-electrode pad 106 which is a preferential light-emission region of which the luminous density is high. Such a limit makes it difficult to enhance the light extraction efficiency of the entire chip. In the meantime, a dotted line of FIG. 1 represents a current path.